There has been used a DC arc furnace as shown in FIGS. 1 and 2 to melt scrap material.
The PC arc furnace comprises a furnace shell 2 with a bottom electrode 1 (anode) at its bottom, a furnace roof 3 adapted to close an upper portion of the furnace shell 2, an upper electrode 4 (cathode) vertically extending through the roof 3 at a center thereof, a dust collecting duct 5 connected to the roof 3, a roof opening-and-closing device 6 which supports the roof 3 for vertical and pivotal movements of the latter and an electrode rising-and-lowering device 7 mounted on the roof opening-and-closing device 6 for vertically moving the upper electrode 4.
The furnace shell 2 is rockably supported by a lower supporting stand 8 through a rocker 9. With the roof 3 being released, the furnace shell 2 is tilted by a tilting drive 10 to take out molten steel 11 in the furnace shell 2 through a spout 12 into a ladle 14 on a ladle carriage 13.
In an operation for melting scrap material 15, the roof opening-and-closing device 6 lifts up the roof 3 and swing it outwardly of the furnace shell 2, thereby opening the top of the furnace shell 2. The scrap material 15 which has been preheated by a preheating device (not shown) arranged at a separate position is charged into the furnace shell 2, using a bucket or the like. Then, the top of the furnace shell 2 is closed with the roof 3. With suction in the furnace shell 2 being made by the dust collecting duct 5, the upper electrode 4 is lowered to a predetermined position and the electrodes 1 and 4 are energized to generate and maintain arc 16 to melt the scrap material 15.
In this case, substantial weight of the scrap material 15 is little in comparison to quantity thereof and a desired quantity of molten metal cannot be obtained only by one melting operation. Therefore, after the completion of one melting operation, the electrodes 1 and 4 are de-energized, the furnace roof 3 is opened and new scrap material is charged into the furnace shell 2. Thus, the scrap material melting operation is repeated several times in the manner described above. Thereafter, the furnace shell 2 is tilted by the drive 10 to pour the molten metal 11 through the spout 12 into the ladle 14.
In the conventional DC arc furnace of the type described above, whenever the scrap material is to be charged into the furnace shell 2, the furnace roof 3 is released. Such release of the roof 3 causes various adverse problems. High-temperature exhaust gases are dispersed outside of the furnace shell 2 so that a considerably large quantity of heat is dissipated to the outside, resulting in a large amount of heat loss. Also a considerably large quantity of dust is spread and noise is generated. When additional scrap material 15 is being charged into the furnace shell 2, the melting operation is shut down so that arc time loss occurs and the temperature of the molten metal drops, resulting in substantial decrease of the melting efficiency.
Moreover, arc 16 between the scrap material 15 and the upper electrode 4 in the furnace shell 2 may be quickly moved over the scrap material so that flicker and/or short circuit occurs, resulting in great voltage fluctuation in the power source system.
In order to overcome the above-mentioned problems, a continuously charged type DC arc furnace as shown in FIG. 3 has been proposed which has a material charging port 17 on a side of the furnace shell 2 or as shown by an imaginary line on the furnace roof 3 and adjacent to the upper electrode 4. The scrap material 15 is continuously charged by a delivery device 17a through the charging port 17 into the furnace shell 2 so as to overcome the above-mentioned problems due to opening and closing of the roof 3 and due to flicker.
However, the continuously charged type DC arc furnace as shown in FIG. 3 has also various problems. Since the scrap material 15 is additionally charged sideways of the upper electrode 4, the added scrap material 15 is always piled up in the furnace shell 2 at an offset position, which makes temperature distribution in the furnace shell 2 asymmetric and requires a separate heating device, or delay in melting time period is caused due to unmelted scrap material, which substantially lowers the melting efficiency. Moreover, since the melting is mainly effected at the offset position in the furnace shell 2 adjacent to a peripheral wall 23 of the furnace, the furnace shell 2 tends to receive local wear.